The need for assay systems which are faster and more efficient has lead to a desire for specific binding agents which have higher affinities with respect to a target ligand. The production and selection of high affinity specific binding agents is not technically difficult; this can be achieved by hybridoma technology or the use of non-mammalian cells such as bacteria which have been modified by genetic engineering to express a specific binding protein. However, the effective recovery of the specific binding agent from the medium (generally a cell culture medium) into which it has been expressed remains an area in which there is considerable room for improvement.
Selective precipitation of the specific binding agent, which is normally a protein such as an immunoglobulin, carries the considerable risk that the protein will be denatured and hence the desired properties of the specific binding agent will be reduced or lost altogether. Affinity purification, in which the desired specific binding agent is selectively adsorbed onto a solid phase material and subsequently eluted, is an alternative. A highly efficient way of extracting the desired specific binding agent from the cell culture medium is to expose the medium to a solid phase, such as a column, onto which is immobilised the target ligand against which the specific binding agent has been raised. However, this results in the specific binding agent becoming very strongly adsorbed onto the solid phase, and the elution procedure necessary to recover it involves extreme conditions such as the use of strong buffers which again are very likely to damage the sensitive reagent. To some extent this risk of damage can be reduced by employing on the solid phase an analogue of the target ligand which is selected such that the specific binding agent has a substantially lower affinity for the analogue than it does for the target ligand. As a result the specific binding agent is less strongly adsorbed onto the solid phase and the elution conditions required to remove it are less severe.